Various proposals have been made to a method of measuring a flow rate of a fluid from a frequency of Karman's vortex streets generated downstream of a vortex street generating member located in a conduit and are disclosed, for example, in Japanese Utility Model Laid-Open No. 54-41665 and Japanese Utility Model Laid-Open No. 57-160625.
Japanese Utility Model Laid-Open No. 54-41665 discloses a method which makes use of the fact that ultrasonic signal waves transmitted across a duct are phase modulated by Karman's vortex streets generated in the fluid. Meanwhile, Japanese Utility Model Laid-Open No. 57-160625 makes use of the fact that the resistance of a thermistor located in a duct and heated with a constant current flow varies in response to Karman's vortex streets.
In both of the conventional methods, the output electric signal is in the form of an analog signal which varies in response to Karman's vortex streets. Thus, where either one of the conventional devices is used, for example, as a suction air amount sensor for fuel injection control of an engine, normally the output electric signal is converted into binary digitized Karman's vortex street pulses for its application. A conventional method of converting an analog signal into such pulses is disclosed, for example, in Japanese Patent Publication No. 58-70131.
By the way, when such a Karman's vortex street flow rate sensor is used as a suction air amount sensor for an engine, it will give rise to following problems as described in part in Japanese Patent Publication No. 58-70131.
In particular, when a throttle valve is positioned at a high opening position, pulsations of air caused by an air sucking action of the engine are transmitted to a suction air path so that the output level of the flow rate sensor is varied significantly in response to such pulsations.
To the contrary, when the throttle valve is positioned at a low opening position, so-called "wind whistles" are produced by suction air passing through the valve at a high speed and have an influence on the Karman's vortex street sensor in the suction air path so that the sensor develops an output in which high frequency noises are overlapped.
When such an output signal is compared with a predetermined voltage on a voltage comparator in order to binary digitize the same to produce Karman's vortex street pulses as disclosed, for example, in above-mentioned Japanese Patent Publication No. 58-70131, a so-called "falling off" of pulses will occur upon high opening of the throttle valve, and on the contrary upon low opening of the valve, the output will involve pulses originating from overlapped high frequency noises.
The present invention has been made to resolve such problems as described above, and it is an object of the present invention to provide a suction air amount measuring device for an engine which always outputs correct pulses of Karman's vortex street irrespective of an operating condition of the engine.
By the way, as digital circuit techniques have been developed in recent years, associated devices such as, for example, microprocessors, gate arrays and memories have been remarkably reduced in price and improved in function.
In view of such circumstances as described above, it is desired for a Karman's vortex street flow rate measuring instrument to be reduced in price, improved in function and reduced in size and weight by constituting signal processing circuitry thereof from digital circuits.
However, the conventional techniques or means described above cannot suitably be formed from digital circuits. For example, if it is intended to form from a digital circuit a phase shifting circuit of the conventional technique which makes a principal part of a signal processing circuit, it gives rise to a problem that a register for a very large number of bits is required.
The present invention has been made to also resolve such a problem as described just above, and it is another object of the present invention to provide a suction air amount measuring device wherein a signal processing system is employed which can be suitably formed from a digital circuit.